Trying to make a custom CSS for using a different image as my desktop vs mobile banner

Hi- I am trying to use an all together different image for my homepage banner image on my Aria site. www.aniwa.co

The desktop image is horizontal based, so for mobile it cuts off and regardless of the coding it will be cut off too much. I have a vertical version of the homepage image that I want to display on mobile. Anyone have a CSS code for a all together different banner image on desktop v mobile?

Thanks!

Danielle

I was able to display a different banner image on mobile with the following code: 

@media only screen and (max-width: 640px) {
       #page-url {
              background-image:url('instertimageurlhere');
         }
    }